Means for securing a driving or driven rotor to a power transmitting shaft are well known in the art of rotating machinery. Such rotors must typically be secured against relative radial, axial, and rotational motion with regard to the shaft and, for certain applications, must further be adapted for disassembly for maintenance or other purposes. One particularly demanding application is the rotor assembly of a gas turbine engine subjected not only to extremely high angular speed, on the order of 20-30,000 rpm, but also to a variation of temperature over the range from ambient up to the working fluid temperature, approximately 2500.degree. F. (1375.degree. C.) or higher.
The attachment of a rotor to a shaft in a gas turbine engine, while located in a relatively cool portion of the engine with respect to the working fluid temperature, nonetheless may reach temperatures up to 500.degree. F. (260.degree. C.) which results in significant thermal expansion of the shaft and rotor components. The radially inner portion of the rotor, termed the hub, is typically sized to fit closely about the power transmitting shaft and further typically includes means, such as a spline, keyway, or the like, for preventing relative rotation between the assembled shaft and rotor. As will be appreciated by those skilled in the art, operation of the rotor assembly at high angular speed induces radial tensile stress in the radially outer or disk portion of the rotor. Such radial tensile stress tends to result in radial expansion of the hub portion of the rotor, opening the radial fit between the rotor hub and the shaft.
Such opening is prevented in the prior art by sizing the outer diameter of the shaft and the corresponding inner diameter of the hub so as to result in a radial interference fit between the assembled components. The use of an interference fit, if properly sized, results in the maintenance of full contact between the rotor hub and the shaft over the entire operating range of the assembly.
Another factor which can cause loosening of the radial fit between the rotor hub and shaft can result from a mismatch of the thermal coefficients of expansion of the rotor material and the shaft material. For rotors of gas turbine engines, the typical material of manufacture is a high strength, temperature resistant nickel-based alloy, such as Waspalloy (a trademark of United Technologies Corporation), which is particularly adapted to withstand the high temperature present in the radially outward fluid path as well as being sufficiently strong at the hub portion to accommodate the high stresses induced by high speed rotation.
Such material, having a thermal coefficient of expansion or alpha equal to 7.35.times.10.sup.-6 at 500.degree. F., may be contrasted with the coefficient of expansion of a carbon steel shaft, 7.15.times.10.sup.-6 at 500.degree. F. Carbon steel, AMS6304 or equivalent, is typically selected for the shaft material based on machinability and cost considerations. As will be apparent to those skilled in the art, the rotor expands at a higher rate than the shaft for an equivalent temperature increase, thereby further increasing the radial dimension of the inner diameter of the hub relative to the outer diameter of the shaft.
The combination of the "thermal loosening" and "rotational loosening" as described above must be considered in determining the ambient temperature, non-rotating radial interference fit between the rotor hub and the shaft. The magnitude of this radial interference fit, as well as the degree of difficulty in assembling and disassembling the rotor and shaft both increase as the engine and rotor are designed to operate at increased temperatures and angular speeds. Assembly/disassembly is further complicated by the need to size both the shaft and rotor to accommodate diametral machining tolerances on the order of .+-.0.002 inches (0.004 mm) to insure that all rotors and shafts of an engine production run will each have at least the minimum cold static interference fit necessary to insure that the assembly will not experience any loss of fit at operating temperatures and speed.
As noted above, assembly can become complicated, particularly in high performance engines which operate at elevated temperatures and/or angular speeds. Such assembly may be accommodated in the production facility by well-known methods of reducing the necessary axial assembly/disassembly force, such as by chilling the shaft and heating the rotor to reduce or eliminate the interference fit during assembly. Such controlled temperatures, as well as the necessary hydraulic and/or mechanical equipment to overcome the interference fit while axially pressing the rotor onto the shaft are frequently not available at remotely located repair facilities, especially field maintenance stations. In such remote locations it is highly desirable to have the ability to assemble/disassemble a rotor assembly using common handtools. Such manual tools presently used are unable to exert the required axial force between the rotor and shaft to overcome the cold static interference fit in a high performance engine, thereby severely compromising the field maintainability of such engines. What is required is a means for securing a rotor to the shaft of a high performance gas turbine engine, or the like, which does not require complicated procedures or machinery to secure or release the rotor and shaft.